He he - several of the Echinodorus have been re-clasdified and are now called Helanthium. Both names still used in the market. Tenellus is a Helanthium, too and so is quadricostatus..............but it's still the same plant, ofcourse.
- so, short answer ; it's the same plant !!
Quite easy plants to care for. Has a tendency to "cramp" newest leaves, cause of the curly appearance. This can be solved by gently pulling them apart.